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Yosemite Nature Notes 47(3) (1978)


PLATE TECTONICS AND YOSEMITE

Cary Avedesian

Most of the stories of the geology of Yosemite restrict themselves to the immediate vicinity of Yosemite. The rocks in the park lend themselves to this type of interpretation but we are located within one of the major structural features of the state of California: the Sierra Nevada.

If we expand on our view to include rocks of the entire state, our story gains interest. It shows Yosemite as a unit in the story of the growth of a continent. The concept of plate tectonics gives us a unifying principle with which to tie it all together. It has struck many geologists as odd that ancient rocks (those of the Precambrian or Early Paleozoic) are not found west of the crest of the Sierra Nevada. These rocks are found in abundance in the mountain ranges east of the Sierra and on the eastern escarpment of the Sierra. Yosemite and the greater part of western California were not a part of the North American continent 300 million years ago. This seems reasonable, as the usual story of Yosemite says that “500 to 150 million years ago most of the west was a shallow sea, and sediments were deposited on the sea floor.” The surprise is that Yosemite was not part of a shallow sea but part of the continental shelf and deep ocean basin.

The earth is divided distinctly into two main parts — continents and ocean basins — as shown in Diagram 1. Often in the past the oceans have lapped high up on the continents, forming shallow inland seas. They can deposit great thicknesses of sediment which bow the continent down. During times of mountain-building the oceans retreat and only inundate the margins, or continental shelves, as shown by the present west coast of North America.

The continental material is made mostly of a lightweight rock called granite. The continents are piles of granitic material swept around like froth by the movement of the heavier oceanic rock called basalt. Since the oceanic and continental rocks are relatively rigid, they are broken into great plates by the
plate tectonics (diagram 1)
[click to enlarge]
forces which set into motion the plastic material of the earth’s mantle. Three types of boundaries are thought to exist between these great plates, as shown in Diagram 2.

What happens when the continental mass is brought to the boundary of two plates? California provides an example of this relationship. From about 200 million years ago to about 80 million years ago, what was to become California was involved in a great subduction zone against the North American plate (Diagram 3). Great slabs of what is now known as the Calaveras and older formations, were scraped downward against the granites of North America. Some of this rock material stuck to the granites as it was moved by. Pieces of the sea floor basalts themselves were scraped off and now show as bands of serpentinite in these rocks. This process was not continuous and probably stopped or slowed for a time while new sediments formed at sea. Once again the process started up and scraped off the younger rocks to the west of the Calaveras Formation. These younger rocks now are known as the Mariposa Slates or tombstone rocks; these may be seen on the way to Yosemite along the Merced road. Much of the sea floor rock material was not scraped off but was drawn deep into the earth where it melted. As the molten sea floor rock moved upward it melted much of the sedimentary material as well, creating granite magma. Again, this was not a continuous process but had many starts and stops, creating first the Western Series of darker igneous (plutonic) rocks, then the El Capitan and related granites, the Sentinel and related granites, and finally the Tuolumne Series of plutonic rocks (Diagram 4).

About 60 to 80 million years ago, a change in the nature of the plate boundaries took place. Both the Pacific and North American plates began to move northwestward in the vicinity of California. The Pacific plate is moving slightly faster than the North American plate (the present boundary seems to be at the San Andreas Fault). Subduction has stopped and rotational movement has begun (refer again to Diagram 2). Both plates are moving the same way but, relative to each other, the Sierra side is moving southeast and the coastal side is moving northwest. This motion should bring Yosemite directly across from Los Angeles in another 40 million years.

At about the time of this change in motion, the ancestral Sierra stood high above the ocean and was firmly attached to the North American plate as new continental material. The North American continent had grown!

A long period of quiet internal forces in the Sierra ensued as this rotation began. The Sierra was acted upon by the external forces of weathering and erosion. Deep soils formed and erosion stripped away most of the rock material above the granites. About 60 million years ago, this area was one of low rolling foothills with granite exposed in places.

A warm tropical climate must have followed for the next 40 million years as deep red soils formed in the foothills. The redwood forests, needing a cooler climate, were still to the north and east. About 10 to 20 million years ago the forces which were pulling California northwestward created tension in the Sierra Volcanoes erupted and renewed uplift occurred. The volcanoes spilled mud and lava into the old stream channels burying them and the gold which had been washed into them.

The climate cooled and the redwood forests began to migrate south and west. New rivers began to flow southwestward cutting across the old river channels. The Merced and Tuolumne Rivers were two of these new rivers. The cutting of the present Yosemite Valley started at this time.

About 5 to 10 million years ago the tension created by the motion of the two great plates on this part of the earth’s crust began to stretch and crack it
plate tectonic zones (diagram 2)
[click to enlarge]

150 million years ago (diagram 3)
[click to enlarge]

90 million years ago (diagram 4)
[click to enlarge]

1 million years ago (diagram 5)
[click to enlarge]
into large blocks. The Sierra is one of these blocks, broken on the east and perhaps on the west. The Sierran block tilted to the west forming a steep eastern face like a great wall facing the state of Nevada, as shown in Diagram 5. Can you imagine the 49ers’ dismay after crossing the rugged Rockies, facing the heat and drought of the desert, to finally be required to cross the great wall of rock of the Sierra? That so many made it is an amazing story. The streams flowed down the long western slopes cutting into the granite. The Merced cut a canyon two-thirds or more of the present depth.

About one to three million years ago, the glacial story begins. The glaciers carved and sculpted the present landscape of Yosemite. They melted completely away as recently as 10,000 years ago. The present glaciers are remnants of the little ice age of 3,000-4,000 years ago.

The eastern boundary of the Sierra is still being pulled upon by the motion of the North American and Pacific plates. Present studies show an upward movement of the Sierran block of 20 to 30 inches per century.

And so the rotational movement and mountain-building started many eons ago continues at the present time.



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